1. Field of the Invention
The present invention relates to a frequency converter unit, and in particular to a frequency converter unit for use in a high-frequency receiver capable of obtaining a high gain and low distortion with reduction of current consumption employing a field-effect transistor (referred to as "FET" hereinafter).
2. Description of the Prior Art
In recent years, there has been a growing demand for a frequency converter provided at a first stage of a receiver unit capable of low distortion with small consumption of current as channels for broadcasting increases in number with reduction in size and improvement in performance of a high-frequency receiver.
FIG. 4 shows a block diagram of a conventional high-frequency receiver. Referring to FIG. 4, the receiver is provided with a receiver antenna 501, band-pass filter units 502 and 506, a first-stage amplifier 503, a frequency mixer (referred to as "mixer" hereinafter) 504, an oscillator 505, an amplifier 507, and an intermediate frequency (referred to as "IF" hereinafter) signal output terminal 508.
The following describes the operation of such a conventional high-frequency receiver circuit constructed as mentioned above.
A high-frequency signal received in the receiver antenna 501 is filtered through the band-pass filter 502 to remove frequency components other than a desired frequency component of the input signal and is then amplified by the first-stage amplifier 503 and the amplified signal is entered to the mixer 504. In the mixer 504, the input signal containing a selected frequency component is converted into an IF signal through mixture with a local oscillation (referred to as "LO" hereinafter) signal. The IF signal is then transmitted through the band-pass filter 506 and the IF amplifier 507 to be finally output from the IF signal output terminal 508.
FIG. 5 shows a circuit diagram of a conventional frequency converter unit. An oscillator section 180 and an oscillator buffer (referred to as "LO buffer" hereinafter) section 181 in FIG. 5 correspond to the oscillator 505 in FIG. 4, while an amplifier section 182 and a mixer section 183 in FIG. 5 correspond to the mixer 504 in FIG. 4. Referring to FIG. 5, the frequency converter circuit is provided with FETs 101 through 111, resistors 120 through 137, capacitors 140 through 147, an inductor 150, variable-capacity diodes 151 and 152, power terminals 160 through 162, IF signal output terminals 163 and 164, an RF signal input terminal 165, and grounding 170.
The following describes the operations of the frequency converter circuit having such components as mentioned above.
The oscillator section 180 oscillates at a resonance frequency of a resonance circuit composed of the variable-capacity diodes 151 and 152 and the inductor 150 and generates an LO signal to be transmitted to the LO buffer section 181. The LO buffer section 181 is provided with a differential amplifier for stabilizing the oscillator section 180 and for generating a balanced signal. The LO signal output from the oscillator section 180 is applied to a gate terminal of the FET 102 constituting a differential pair in the LO buffer section 181. The input LO signal is converted into a balanced LO signal including a phase difference in the LO buffer section 181 so as to be transmitted to the mixer section 183. In the meantime, an RF signal input to a gate terminal (i.e., the RF signal input terminal 165) of the FET 109 constituting the differential pair of the amplifier section 182 is converted into a balanced RF signal in the amplifier section 182. The balanced RF signal output from the amplifier section 182 is entered to the mixer section 183. The balanced LO signal and the balanced RF signal are subject to frequency conversion in the mixer section 183 so as to form a balanced IF signal. The balanced IF signal containing the sum and difference in frequency of the balanced LO and RF signals is output from the IF signal output terminals 163 and 164. As a type of the mixer, there is used a double-balanced mixer having an improved secondary distortion characteristic preventing the LO signal from leaking to the RF terminal. The balanced IF signal output from the mixer 183 is converted into an unbalanced IF signal through synthesizing both the signals in a subsequent balanced-to-unbalanced signal converter and the unbalanced IF signal is then transmitted to an IF signal processing section.
The above-mentioned high-frequency receiver is required to have a high gain performance, low distortion characteristic, and a small consumption of current. Among those factors, the gain depends on the K value and the mutual conductance gm of the FET used in the circuit, while the distortion characteristic depends on the linearity of the input-output characteristic of the FET. In other words, the greater the K value and gm of the FET are, the higher the gain results. While the better the linearity is, the less the distortion results. The drain current (Ids) of the FET can be expressed approximately as follows. EQU Ids=K(Vgs-Vth).sup.2 ( 1)
where Vgs represents the voltage across the gate and the source, Vth represents the threshold level of the FET, K represents a parameter measured in microamperes per square volt of the FET. In the equation (1), gm can be derived by differentiating both the members by Vgs as follows. EQU gm=2K(Vgs-Vth) (2)
As obvious from the equation (2), gm is proportional to the K value and Vgs. However, in general, in order to increase the K value of the FET formed through a selective ion incorporation, the threshold level Vth is required to be shallow, resulting in that the input-output linearity is deteriorated so that the distortion characteristic conversely degrade. In order to improve the input-output linearity, the K value is required to be small to moderate the gm characteristic with respect to Vgs in the vicinity of the operating point, which results in increasing the absolute value of the threshold voltage Vth to increase the consumption current.
As obvious from the above facts, there is a reciprocality between the gain and the distortion characteristic, and there is also a reciprocality between the distortion characteristic and the consumption current. Therefore, when designing a circuit employing FETs having the same characteristic, it is compelled to select a reasonable point compromising between those characteristics. In the conventional example in FIG. 5, an FET having a characteristic as shown in FIG. 6 is employed. The FET having the characteristic in FIG. 6 has a K value of 170 mA/V.sup.2. mm, a threshold voltage of -0.6 V, and a mutual conductance gm of 160 mS/mm.
However, in the conventional circuit construction as mentioned above, for the reason that FETs having the same characteristic have been employed in the oscillator section and in the amplifier section, there has been a problem that the distortion characteristic of the amplifier section deteriorates when an FET having a large K value is used making great account of oscillator output, or otherwise the consumption current increases and the oscillator output reduces when an FET having a small K value and a large threshold voltage is used making great account of the distortion characteristic of the amplifier section.